ci: ostree_rs_ext job uses archived ostree-rs-ext repository

Problem

The ostree_rs_ext GHA job clones github.com/ostreedev/ostree-rs-ext, which is archived (points to containers/bootc).

The job builds skopeo, compiles ostree-rs-ext with cargo, and runs its tests. It is merge-blocking via Total Success.

Review context

CI Switch tracking issue #2633 lists ostree-rs-ext under "Not yet in TMT" with an open question: "still needed?"

Discussion there:

• mtrmac: integration test against the primary user still seems valuable; RPM reverse-dependency testing might be an alternative
• cgwalters: Testing Farm may support reverse-dependency testing — triggering bootc tests on skopeo changes upstream and downstream
• lsm5: linked TMT work in PR #2640

Draft PR #2640 adds TMT coverage for ostree-rs-ext but still uses the archived repo, not bootc.

Related

• #2633 — CI Switch: Cirrus → TMT
• PR #2640 — TMT work in progress
• GHA job: ostree_rs_ext in .github/workflows/ci.yml